1. Field of Invention
This invention relates to novel polymers containing appended carboxyl groups and a process for the preparation thereof. More particularly, this invention relates to a method for appending carboxyl groups onto polymers by reacting a mixture of maleic anhydride, a free radical catalyst and an additive, with a polymer or copolymer, above its melting point, in the absence of a solvent, and to modified polymer products obtained thereby.
2. Description of the Prior Art
The reaction of maleic anhydride with polymers is well known in the art. The thermal "ene" reaction between an unsaturated polymer and maleic anhydride has been widely used. U.S. Pat. No. 2,973,344 discloses the preparation of modified polyolefins by heating low pressure polyalkene polymers "known to possess non-terminal unsaturation" with maleic anhydride, in the presence or absence of either an inert hydrocarbon solvent and/or a peroxide catalyst. U.S. Pat. No. 3,884,882 discloses the preparation of "adducts" of maleic anhydride and unsaturated elastomeric ethylenepropylene-unconjugated diene terpolymer by reaction at elevated temperatures in the absence of a radical catalyst, e.g. an organic peroxide. The presence of a radical catalyst results in the crosslinking of the elastomer.
The grafting of maleic anhydride onto saturated polymers in the presence of free radicals, either generated by shearing or by heating free radical precursors such as organic peroxides, is also well known in the art. However, reaction in the absence of a solvent results in crosslinking, e.g. in polyethylene or ethylenepropylene copolymer, or degradation, e.g. in polypropylene. U.S. Pat. No. 3,236,917 discloses that the grafting of maleic anhydride onto saturated copolymers of ethylene and an .alpha.-olefin, in the presence of a radical initiator at elevated temperatures and in the absence of a solvent, results in the grafting of maleic anhydride onto two chains, thereby crosslinking the copolymer. U.S. Pat. No. 3,862,265 discloses the controlled degradation of polyolefins in an extruder in the presence of a radical catalyst and the concurrent grafting of maleic anhydride onto the base polymer.
Gabara and Porejko (Journal of Polymer Science, A-1, 5, 1547 (1967) reported that when maleic anhydride is grafted onto low density polyethylene film suspended in an acetic anhydride solution containing a free radical catalyst, the film becomes crosslinked. However, reaction in a solvent such as xylene results in appendage of maleic anhydride without crosslinking the polyethylene (S. Parejko, W. Gabara and J. Kulesza, Journal of Polymer Science, A-1, 5, 1563 (1967); D. Braun and U. Eisenlohr, Die Angewandte Makromolekulare Chemie, 55, 43 (1976). U.S. Pat. No. 3,873,643 discloses that after mixing high density polyethylene with maleic anhydride in the presence of a radical catalyst in a Brabender Plasticorder at elevated temperatures, the melt index of the polymer is significantly reduced.
The reaction of molten isotactic polypropylene with maleic anhydride in the presence of a free radical catalyst in a Brabender Plasticorder (F. Ide, K. Kamada and A. Hasegawa, Kobunshi Kagaku, 25, 107 (1968) or in an extruder (I. Sasaki and F. Ide, Kobunshi Ronbunshu, 38, 67 (1981) results in extensive degradation and a decrease in the intrinsic viscosity of the polymer. When the reaction of isotactic polypropylene with maleic anhydride is conducted in solution in xylene in the presence of a peroxide, the extent of degradation is decreased (F. Ide, A. Hasegawa and T. Kodama, Kobunshi Kagaku, 25, 167 (1968).
The use of solvents to prevent the crosslinking or degradation of the polymer which occurs during the reaction with maleic anhydride in the presence of a radical catalyst, requires the use of equipment for solution reactions and the separation and recovery of the solvent as well as the polymer.
Although reduced degradation occurs in the reaction of crystalline polypropylene in particulate form, below the melting point of the polymer, with vapors of maleic anhydride in the presence of a peroxide, as disclosed in U.S. Pat. No. 3,414,551, the extent of reaction is limited by the surface area of the particulate polymer.
The present invention is directed towards an improved process for the preparation of maleic anhydride-modified polymers in the absence of solvents.